In manufacturing a terminal bushing of the above type, a mold is provided that surrounds the central conductor and forms about the central conductor a mold cavity that has pinch points at the ends of the mold cavity where the mold and the central conductor interface. Liquid polymer under high pressures is forced into the mold cavity to fill it and is subsequently cooled to solidify the polymer and form a molded shell conforming to the shape of the mold cavity. It is important that this molded shell be free of voids that could cause corona or fluid leakage problems in the finished product, i.e., the terminal bushing. In order to obtain the desired freedom from such voids, it is important that the above-described pinch points at the ends of the mold cavity be highly restricted so as to limit to a very low value the amount of pressurized liquid polymer that is ejected from the mold via the pinch points during the molding process.
In order to produce the desired high restriction at the pinch points of the mold, it is important that the outer dimensions of the central conductor closely match the internal dimensions of the mold at the pinch points. One way of achieving the desired close match is to provide a central conductor that initially is slightly oversized at the pinch points and to then carefully machine the conductor at the pinch points to the precise size desired. This approach has the disadvantage of being unduly expensive, labor-intensive, and time-consuming.
An object of our invention is to construct the molded-shell bushing in such a way that there is no need to machine the central conductor in order to limit to a very low value the amount of liquid polymer ejected from the mold at its pinch points during the molding operation.
Another object is to provide a terminal bushing that comprises a polymeric insulating shell molded about a central conductor that is characterized by an insulating shell that is essentially free of voids that could cause corona or fluid leakage problems in the bushing.
In many terminal bushings at the outer end of the central conductor there is coupling structure that is used for connecting an external conductor to the central conductor. This coupling structure sometimes takes the form of a clamp including a basket or a blade against which the external conductor is bolted. The basket or the blade must be joined to the central conductor, and this usually results in a multi-part structure that is relatively expensive to fabricate.
Another object of our invention is construct the bushing in such a way that the central conductor and the above described basket or blade can be produced as a single unitary part that contains no joints requiring assembly.
The terminal bushing is a component of electrical apparatus and extends through an opening in a wall of the apparatus. The polymer insulating shell of the bushing has an external thread on its outer surface that meshes with a sheet metal nut that is tightened on the shell to press against the wall and thereby to hold the bushing securely in place within the wall opening.
Another object of our invention is to provide for this bushing mounting application a sheet-metal nut that has a strong resistance to loosening in response to vibrations and the like and which has a reduced tendency to damage the external threads on the bushing shell when tightened to press against the wall on which the bushing is mounted.